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Uh, after having read all my mail, I see what prompted
your enquiry.   There is a point to what KMP has been
saying, which none of the discussion seems to have
noticed, that by making DISPLACE to be not very hairy
   1) it can serve as the canonical RPLACA and RPLACD
      for **any** of the various macro-displacing schemes
      which have abounded in the past two years - there is
      the danger of one complicated macro-scheme thinking 
      that it is the *only* choice. 
   2) it can be the canonical place where the question of 
      clobbering a "pure" cell must be handled.  RPLACA and 
      RPLACD are open-coded by maclisp, and one probably
      wouldn't want the same  pure-page-trap action/error to 
      be done for them as for DISPLACE.  A close-compiled
      function like DISPLACE can rather easily do a "purep?" 
      check by looking into the maclisp internal tables.
No, this purity check hasn't yet been done, but as you say, 
"we should reason together about what the right thing is and 
what should be done about it."
    By the bye, I assume that everyone who has pitched in
a opinion so far is on one of the two mailing lists, BUG-LISP
and NIL.  So it you didn't get this note, and had a strong
opinion, you should complain (I suppose).